Screwdown control system for rolling mills

ABSTRACT

The velocity of a load is controlled by an electrohydraulic servoactuator including hydraulic actuator means for positioning said load, and electrical input servovalve means for controlling the flow of fluid with respect to said actuator means. When zero velocity is desired a position holding means of an electrical nature is activated to produce an input signal for said servovalve means to counteract the tendency for load position variation.

United States Patent [1 1 Dark [ 511 Feb. 27, 1973 SCREWDOWN CONTROL SYSTEM 54 3,587,279 6/1971 Sendzimir ..72/240 3,178,919 4/1965 Varner ..72/9

[75] Inventor: Patrick M. Dark, East Aurora, primary Mi s M h N.Y. Attorney-Sommer, Weber & Gastel [73] Assignee: Moog, Inc., East Aurora, NY. 57 ABS R l. l l s T ACT [22] Filed: Oct. 26, 1971 1 The velocity of a load is controlled by an elec- [21] Appl. No.: 192,486 trohydraulic servoactuator including hydraulic actuator means for positioning said load, and electrical input servovalve means for controlling the flow of [52] US. Cl. ..72/8, 72/10, 7722/;169, fluid with respect to Said actuator means when zero velocity is desired a position holding means of an elec- [51] 11.11. C]. ..B21b 37/00 trical nature is activated to produce an input signal for Field Of Search 10, 16, id servovalve means to counteract th t d f 12 load position variation. [56] References Cited 14 Claims, 1 Drawing Figure UNITED STATES PATENTS 3,069,605 12/1962 Wallace ..72/16 X "i 93 RESOLVER I 9| 1 7 us 179 92 k REFERENCE SM 2 f I 82 H9 COUNTER COSINE E CLOCK |2| s4 2 (g j I; l [20 COMMAND ONE 78 75 1 23 courgan l{3 2 SHOT -126 L J1 uo 90 I22 x/ F LI IOB Q09 1 PHASE FLOP #04 'BI ANGLE I f DETECTOR I03 I02 I FLOP NULL DETECTOR 96 DOWNSTREAM 5 TRIM TACHOMETER SERVO AMPLIFIER HYDRAUL lC ACTUATOR SOREWDOWN CONTROL SYSTEM FOR ROLLING MILLS BACKGROUND OF THE INVENTION predetermined position is reached at which it is desired 1 to hold the element. The prior art has not provided arrangements which are fully satisfactory to achieve such objectives.

For a first example, in the control of a grinding machine, it may be desired to select manually the feedrate for the grinding element, and then stop and hold such element at preselected positions.

A second example is the control of a drilling machine wherein the drill feed velocity is present with stop and hold of the drill at limit switch positions.

A third example is the velocity control with zero drift of a shot cylinder in a die casting machine.

A fourth example is the velocity and hold control of the injectant piston that forms the parison in a plastic injection molding machine.

Still another example may be found in a screwdown control system for rolling mills. This example will be now considered in greater detail.

In mills for rolling sheet or strip metal, work rolls are provided which are spaced apart the desired distance so that the workpiece is reduced in thickness each time it passes between the rolls. The workpiece typically ex tends between coilers arranged on either side of the mill. Automatic gauge control means are provided between the coiler and mill on each side of the work rolls so that the thickness or gauge of the workpiece can be measured as it enters the roll bite and also can be measured on the exiting side of the work rolls. In such a mill of one type the work rolls are backed up by intermediate rolls and these in turn are backed up by backing bearing assemblies having eccentric mountings on a frame such that a rotation of these eccentrically mounted assemblies will cause a change in the relative position of the work rolls. Typically the eccentric mountings have toothed sectors which engage with a double toothed rack moved by a fluid operated actuator, the flow of fluid with respect to which may be under control of an electrohydraulic servovalve. A rolling mill of this general type is disclosed in US. Pat. No. 2,479,974 to Sendzimir, for example.

In control of such a rolling mill, use of an automatic gauge control system requires the screwdown control in order to maintain a position of the work rolls for zero automatic gauge control output. An automatic control gauge control output is an electrical error signal in response to the difference between a preset thickness for the workpiece and the actual thickness thereof at a location removed from the roll bite. When an automatic gauge control output is created as a result, for example, of a selectively determined new preset thickness, the screwdown control must be operated to move the rolls to a new position and to hold such new position as the automatic gauge control output reduces to zero because of this movement.

Prior art screwdown control systems performed this task with directional solenoid valves controlling flow to a hydraulic motor. The motor shaft positioned a mechanical-input valve which was arranged for position follow-up by the pistons that provided the screwdown actuation. This arrangement was not satisfactory due to leakage and friction of the hydraulic motor. I

More recently but stillpart of the prior art, and to produce better response and holding accuracy, the screwdown control. systems have employed electrohydraulic servovalves controlling the actuators but arranged in a closed position loop control to maintain holding accuracy. A command transducer commanded the desired position. More specifically, the automatic gauge control output or amanually determined input energized an electrical instrument motor which in turn drove the command transducer such as a linear variable differential transformer or other suitable type of position transducer to a new position. The actuator position loop then followed its command to the new position. Sometimes a velocity loop was created around the instrument motor. Position drift, integration lineari-. ty and response of the screwdown control system were all degraded by the aforementioned instrument motorcommand transducer operation.

SUMMARY OF THE INVENTION This invention relates to an electrohydraulic servovalve for controlling the velocity of a load, and more particularly to one which has a zero drift capability, that is a velocity servoactuator with a position holding mode.

In its illustrated application, the present invention relates to an improved screwdown control system for rolling mills of the type described hereinabove which involves providing a closed loop velocity control around the combination of the electrohydraulic servovalve means and actuator means, thus becoming of itself the integrator. Automatic gauge control outputs and manual inputs are set directly as commands to this velocity loop. With no intervening hydraulic or electrical motor or other instrument driving mechanisms, as in the prior art arrangements, the improved system of the present invention provides an optimum response. Integration linearity of the improved system is excellent due to the wide speed range of the electrohydraulic velocity loop.

Another feature of the present invention is to avoid position drift by providing a position loop in addition to the velocity loop but only inuse when there is no velocity command from the automatic gauge control or manual input. In other words, the position loop is operative when the integrating velocity loop is required to hold a given position of the work rolls.

Accordingly, in its broader aspect the present invention is adapted to any suitable application, the primary objective of the invention is to provide an electrohydraulic zero drift velocity servoactuator.

In its narrower aspect, where the present invention is especially and advantageously suited for embodiment in an improved screwdown control system for rolling mills, an important object of the invention is to provide velocity mode means operatively associated with the electrohydraulic servo means to provide a velocity loop for producing a correction of the work rolls at a velocity proportional to the magnitude of an electrical command signal regardless of derivation of such signal from automatic gauge control means or manual control means.

Still viewing the present invention in such narrower aspect, a further object of the invention is to provide such an improved screwdown control system which also includes position holding mode means operatively associated with said velocity loop to provide a position holding loop for holding the position of the work rolls when the command signal is zero.

In summary, the present invention provides: in its broader aspect, an electrohydraulic servoactuator for controlling the velocity of a load, comprising hydraulic actuator means for positioning said load, electrical input servovalve means for controlling the flow of fluid with respect to said actuator means, and zero velocity position holding means including phase generating means responsive to the output motion of said actuator means and having at least two phase windings and a pick-up winding producing a signal output the phase of which varies with load position; and in its narrower aspect when applied to a rolling mill including work rolls between which a workpiece is moved for reduction of its thickness, firstly, a velocity servoactuator comprising screwdown hydraulic actuator means for positioning said rolls and electrohydraulic servovalve means for controlling the flow of fluid with respect to said actuator means and controlled by an electrical command signal selectively derived from either automatic gauge control means or manual control means, such actuator and servovalve means being so operatively associated as to provide a velocity loop for producing a correction of the position of said rolls at a velocity proportional to the magnitude of said command signal, and secondly, such a velocity servoactuator further comprising position holding means operatively associated with said velocity loop to provide a position holding loop for holding the position of said rolls. when said command signal is zero.

Other objectives, advantages and features of the present invention will be apparent from the following detailed description of a preferred embodiment disclosed in the accompanying drawing which comprises only a single figure representing a typical rolling mill schematically with its associated coilers and automatic gauge control means and illustrating schematically in association therewith the improved screwdown control system constructed in accordance with the principles of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT A type 1-2-3-4 Sendzimir cold rolling mill is illustrated in the drawing as including a mill housing and a roll cluster containing eight backing bearing assemblies 1 1-18 and twelve rolls 19-30. Four rolls 19-22 are driven by suitable means (not shown) while two work rolls 23 and 24 are friction driven by intermediate rolls 25-28. A second intermediate roll 29 is shown arranged between driven rolls l9 and 20, and a similar second intermediate roll 30 is shown arranged between driven rolls 21 and 22. Work rolls 23 and 24 have a vertical spacing between their opposing peripheries to permit the passage through such space of a workpiece W the ends of which are coiled on coilers 31 of known construction after passing over guide rollers 32.

Screwdown control means are shown as operatively associated with backing bearing assemblies 12 and 13. Such means are disclosed schematically as comprising a double toothed gear rack 33 operatively associated with two sectors on the corresponding ends of backing bearing assemblies 12 and 13, and hydraulic actuator means 34 including a cylinder 35, a piston 36 mounted on one end of a piston rod 38 the opposite end of which is suitable connected to gear rack 33. This cylinder 35 is suitably mounted rigidly on mill housing 10. The space above piston 36 within the actuator cylinder provides an upper actuating chamber 39, and the space below this piston within the cylinder provides a lower actuating chamber 40.

It will be understood that although a single hydraulic actuator means 34 is illustrated in the drawing there will be one such means at each end of the mill so that in actuality a air of such hydraulic actuator means will be provided and coupled in parallel.

Electrohydraulic servovalve means 41 of any suitable construction such as disclosed in United States Patent No. 3,023,782, for example, is shown operatively associated with hydraulic actuator means 34. Servovalve 41 is shown as having a pressure inlet conduit 42, a return conduit 43, a first conduit 44 connecting one of its control ports to actuating chamber 39 and a second conduit 45 connecting the other of its control ports to actuating chamber 40. Conduit 42 supplies pressurized hydraulic fluid from a suitable source (not shown), and conduit 43 returns fluid to such source. The servovalve 41 includes an electrical force motor, a torque motor, for example, having a winding or coil such as represented schematically by winding 46 shown as grounded at one end.

The numeral 48 represents an amplifier having an electrical input conductor 49 and an electrical output conductor 50 which leads to and is connected to the ungrounded end of coil 46. More specifically, amplifier 48 is shown as an integrating amplifier by reason of feedback capaciter 48 arranged in an electrical conductor 48" at its opposite ends connected to conductors 49 and 50.

Automatic gauge control means are shown schematically as including a first gauge or thickness sensor 51 and a second gauge or thickness sensor 52. These sensors are shown as connected via electrical conductors 53 and 54, respectively, to a control 55 arranged to provide an electrical anticipation output signal conducted via electrical conductor 56. This conductor is shown as electrically connected to an amplifier 58 which is also arranged to receive an electrical downstream trim signal input through an electrical conductor 59 leading from control 55.

The output of amplifier 58 is shown as connected electrically via a conductor 60 to one contact 61 of a switch 62 which also has a second contact 63 and a throw 64. Contact 63 is shown as connected electrically to a manually set potentiometer 65 via a conductor 66. The throw 64 of switch 62 is shown as electrically connected via a conductor 67 to one end of a resistor 68 the other end of which is connected electrically to conductor 49.

In accordance with the present invention, velocity mode means are operatively associated with servovalve 41 to provide a velocity loop for producing a correction of the position of work rolls 23 and 24 at a velocity proportional to a magnitude of the automatic gauge control command signal leaving amplifier 58 via conductor 60, when the mill is set up for operation on an auto matic gauge control basis. If, on the other hand, the mill is set up for manual control, switch 62 is selectively operated so that its throw 64 contacts terminal 63 so as to permit a manually set electrical command signal determined by the setting of potentiometer 65, designated manual trim in the drawing, to be transmitted via conductor 66.

Adverting to the velocity mode means, the same is shown as comprising velocity sensing means 69 such as a tachometer having a rotor 70 and a pick-up winding 69'. This rotor is operatively associated with the output motion of actuator means 34 as represented by the broken line 71 leading from the tachometer rotor to piston rod 38. The electrical signal generated by pick up winding 69' when rotor 70 is operated is conducted via conductor 72 having a resistor 73 therein to a summing point 74 on conductor 49. In this manner the tachometer means 69 acts as a velocity sensing means arranged to produce an electrical velocity signal responsive to the velocity of the output motion of actuator means 34.

Any automatic gauge control command signal, or manual trim signal, transmitted via conductor 67 is summed with the electrical velocity signal transmitted by conductor 72, such summing occuring at point 74 to provide an input signal for amplifier 48 the output of which via conductor 50 controls the energization of servovalve coil 46. This servovalve controls, proportionately to the electrical signal input to this valve, a hydraulic fluid flow in conduits 44 and 45 with respect to hydraulic actuator meanS 34 so as to operate actuator means 34 to control screwdown and the relative position of work rolls 23 and 24.

The velocity sensing means 69, the servovalve means 41 and the actuator means 34 collectively constitute velocity servoactuator means which directly position the screwdown piston 36 in response to electrical command signals transmitted via conductor 67.

Also in accordance with the present invention, position holding mode means are provided which are operatively associated with the velocity loop to provide a position holding loop for holding the position of work rolls 23 and 24 when the command signal in conductor 67 is zero.

Such position holding mode means is shown as including resolver means 75 having a stator including two phase windings 76 and 77 shown as connected at one end by a grounded conductor 78. The resolver means is also shown as having a rotor 79 including a pick-up winding 80 shown as grounded at one end. This rotor 79 is operatively responsive to output motion of hydraulic actuator means 34. For this purpose, such rotor 79 is shown as mechanically coupled to tachometer rotor 70 so as to rotate therewith, such mechanical connection being represented by the broken line 81.

The position holding mode means is also shown as including high frequency clock mean 82, reference counter means 83, command counter means 84, null detector means 85, phase angle detector means 86, first flip-flop logic means 88, second flip-flop logic means 89, one-shot means 90, and sampling type demodulator means 91. A conductor 92 is shown as electrically connecting the ungrounded end of rotor pick-up winding to sampling demodulator means 91. The resolver rotor winding signal in conductor 92 is likewise electrically conducted, via conductor 93, to phase angle detector means 86. The output of this means 86 is conducted via electrical conductor 94 to an input terminal 95 of flip-flop logic means 88.

Null detector means 85 has an electrical input conductor 96 connected at one end to conductor 67, such connection being indicated at point 98. The output of null detector means 85 is conducted electrically via conductor 99 to a second input terminal 100 on flipflop logic means 88 which also has an output terminal 101. This output terminal 101 is electrically connected via a conductor 102 to an input terminal 103 on flipflop logic means 89. This means 89 has an output terminal 104 connected electrically via conductor 105 to an input terminal 106 on command counter means 84. A branch conductor 108 electrically connects conductor 105 to an input terminal 109 on one-shot means 90. Means 84 also has another input terminal 110 and an output terminal 111. Terminal 111 is connected electrically to another input terminal 112 on one-shot means 90 via a conductor 1 13.

Flip-flop logic means 89 is shown as having another input terminal 114. Reference counter means 83 is shown as having an input terminal 115 and two output terminals 116 and 118. Terminal 115 is shown as connected electrically via a conductor 119 to clock means 82. This conductor 119 is shown as having a branch conductor 120 connected electrically at one end to conductor 1 19 as indicated at point 121 and at its other end to input terminal 110 on command counter means 84. A second branch line 122 is shown as connected electrically at one end to branch line 120 at point 123 and at its other end to input terminal 114 on flip-flop logic means 89.

Reference counter means 83 has one of its output terminals 116 arranged to produce a sine signal which is conducted electrically via conductor 124 to the ungrounded end of phase winding 76. The other output terminal 1 18 of reference counter means 83 is shown as having one end of an electrical conductor 125 connected to it and the other end to the ungrounded end of phase winding 77 for conducting the cosine signal output of the reference counter means.

One-shot means 90 has an output terminal 126 connected electrically via a conductor 128 to sampling demodulator means 91. Such means is shown schematically as being in the nature of a switch element adapted to make or break connection with a contact 129. This contact 129 is shown as connected electrically via conductor 130 through resistor 131 to a second summing point 132 on input conductor 49. A between samples" memory means is shown schematically by capacitor 133 operatively associated with contact 129 through connection point 134.

The high frequency clock means 82 may comprise a crystal oscillator putting out a square wave varying from an electrical 0 state to an electrical 1 state at a frequency of 1 megahertz.

The reference counter means 83 may count down by a binary factor of 2 or 2048 so as to provide an excitation frequency'of approximately 488 hertz for resolver 75.

The command counter means 84 has the same binary factor as the reference counter means 83, so it will put out a square wave at the same reduced frequency, namely 488 hertz for the example stated above, although such signal may have a ditferent phase from that of the reference counter means 83.

The one-shot means 90 converts the command counter square wave output signal to a sharp pulse once per cycle.

The phase angle detector means 86 detects the correct zero crossings of the signal output of resolver 75.

The null detector means 85 is an on or off device, on producing electrical and off producing electrical l. Functionally, when null detector means 85 senses a command signal via conductor 96 which is other than null, all of the position mode holding circuitry is shut off. Thus as electrical 0 output from such means 85 conducted via conductor 99 creates an electrical 0 output at terminal 101 of first flip-flop logic means 88, and in turn an electrical 0 output at terminal 104 of second flip-flop logic means 89. This last output via conductor 105 shuts off command counter means 84.

When the command signal in conductor 67 sensed by conductor 96 reaches null, the output of null detector means 85 goes from electrical 0 to electrical 1 after a short time delay provided by these means, such as 100 milliseconds. This delay avoids transient use of the position holding mode. After this time delay, via conductor 99, the input at terminal 100 goes from electrical O to electrical l, but this by itself produces no change at output terminal 101.

When the phase as sensed by phase angle detector means 86 goes to the correct zero phase crossing, the output of this means transmitted via conductor 94 changes the input at terminal 95 from-electrical l to electrical 0. With terminal 100 already at electrical l, the electrical transition of l to 0 at terminal 95 changes the output at terminal 101 from electrical 0 to electrical 1. Inside flip-flop logic means 88 there is a feedback from output terminal 101 to an input such as terminal 95 that will hold the output of this means locked to an electrical 1 until the input at terminal 100 changes state.

Flip-flop logic means 89 now having an electrical 1 input at terminal 103 transmitted via conductor 102, will produce no change at the output terminal 104 of this means until the input at terminal 114 goes from electrical l to electrical 0 at which time the the output terminal 104 will change to an electrical l. The function of means 89 is to start the command counter means 84 at a favorable instant of clock output voltage, sensed at input terminal 114, as when electrical 1 goes to electrical 0. Means 89 also has feedback from output terminal 104 to an input such as terminal 114 to hold the output of this means locked to an electrical 1 until the input at terminal 103 changes state. A state of electrical l at output terminal 104 is conducted via conductor 105 to terminal 106 of command counter means 84 and thereby turns on this counter means.

The state of electrical 1 in conductor 105 is transmitted via branch conductor 108 to input terminal 109 of one-shot means 90 to render such means operative to function to give an initial sampling pulse before completion of the first cycle out of the command counter means 84.

The sharp pulse out of one-shot means triggers-on the sample demodulator means 91. This demodulator connects momentarily the resolver rotor signal in conductor 92 to the summing point 132 via conductor and resistor 131. Since but a few microseconds will have elapsed since the phase angle detector means 86 senses the correct zero crossing, the phase of the signal in conductor 92 will be essentially unchanged, so the signal amplitude will still be zero at the moment first sensed by the sample demodulator.

Now from this time on, while the position holding circuitry remains activated via an electrical i in conductor 99, the command counter will produce an output at the reduced frequency 488 hertz. At each succeeding cycle of this frequency the one-shot means 90 will trigger the sample demodulator 91. If the resolver rotor has not moved, then the signal in conductor 92 will be zero when the sampling demodulator closes.

If, however, the rotor 79 moves due to movement of actuator piston 36, then the phase of the resolver output signal will shift with respect to the phase of the command counter output signal. This will cause the amplitude of the rotor signal at the moment of sensing by the sample demodulator 91 to no longer be zero. Instead it will be either plus or minus depending upon the direction of rotation of the resolver rotor. This plus or minus signal is then passed by conductor 130 to summing point 132 where it causes a change in the signal output of amplifier 48, and a corresponding change in current in the servovalve winding 46. The servovalve 41 responds by producing fluid flow in conduits 44, 45 in the direction to cause piston 36 to move back towards its previous position.

The sample demodulator 91 continues to detect this phase 1 difference once each cycle of the reduced frequency (that is, 488 times per second). The positive or negative pulses out of the sampling demodulator at point 134 are sustained or held during the time between pulses by capacitor memory means 133, thereby providing a dc, polarity-sensitive command to servovalve 41 via resistor 131, summing point 132, and amplifier 48.

The electrical output of resolver 75 is thus a voltage the phase of which represents position. This phase is compared with the phase output of a command counter or second frequency divider driven by the same clock down to the same carrier frequency in a phase sensitive demodulator in such a manner as to produce a proportiOnal polarity sensitive dc output that is zero when such phases are equal. The dc output is summed to the velocity loop, forming an outer position loop for accurate position holding at the point where the position holding circuitry is activated by the condition of zero velocity command in conductor 67. A sampling type demodulator is preferred to reduce the symmetry requirements of resolver output wave shape.

When a command signal appears, either from the automatic gauge control means or manual trim means, this signal is sensed by the null detector which shuts off the command counter and sampling demodulator. Thus while such a command signal exists, the position loop is disconnected. Very shortly after a command signal ceases to exist, as when the actuator comes to zero speed, the newly established zero phase of the resolver, that is, the first stable zero crossing from the resolver rotor winding, is determined by the phase angle detector. This occurrence allows the command counter to start, thus reconnecting the position loop at the newly established null or holding point.

The various electrical and electronic components discussed hereinabove are well known to those skilled in the art and therefore require no more specific description.

What is claimed is:

1. In an electrohydraulic servoactuator for controlling the velocity of a load, including hydraulic actuator means for positioning said load and electrical input servovalve means for controlling the flow of fluid with respect to said actuator means, the improvement which comprises zero velocity position holding means including phase generating means movable in response to the output motion of said actuator means, said phase generating means including at least two phase windings and a pick-up winding having a signal output whose phase varies with load position.

2. An electrohydraulic velocity servoactuator according to claim 1 which further comprises tachometer means including a tachometer rotor rotated in response to the output motion of said actuator means, and said position holding means includes resolver means having a resolver rotor and a stator, said resolver rotor also being rotated in response to the output motion of said actuator means said rotor including a pickup winding, said stator including two phase windings, high frequency clock means, reference counter means operatively associated with said clock means and said two phase windings and arranged to be driven by said clock means to provide a lower carrier frequency with a two phase output which energizes said two phase windings, command counter means operatively associated with said clock means and arranged to produce a phase output at the same frequency as said carrier frequency, sampling demodulator means comparing the phase of the electrical output of said pick-up winding with said phase output of said command counter means and arranged to produce a polarity-sensitive output that is proportional to the difference between such phases but zero when said phase are equal, means arranged to feed said polarity-sensitive output to said servovalve means, and shut-off means arranged to shut off said command counter means when the command signal exists and thereby provide no such proportional polarity-sensitive output.

3. An electrohydraulic velocity servoactuator according to claim 1 wherein said position holding means includes resolver means having a stator including two phase windings and also having a rotor operatively responsive to output motion of said actuator means and including a pick-up winding, high frequency clock means, reference counter means operatively associated with said clock means and said two phase windings and arranged to be driven by said clock means to provide a lower carrier frequency with a two phase output which energizes said two phase windings, command counter means operatively associated with said clock means and arranged to produce an output at the same frequency as said carrier frequency but having a phase differing by any cycle of said clock means, phase sensitive demodulator means comparing the phase of the electrical output of said pick-up winding with said phase output of said command counter means and arranged to produce a polarity-sensitive output that is proportional to the difference between such phases, means arranged to feed said polarity-sensitive output to said servovalve means, and shut-off means arranged to shut off said command counter means when a command signal exists and thereby provide no such proportional polarity-sensitive output.

4. An electrohydraulic velocity servoactuator according to claim 3 which further comprises one shot means operatively associated with said command counter means and arranged to produce an output pulse having the same phase and frequency as the output from said command counter means, and wherein said phase sensitive demodulator means includes sampling demodulator means controlled by said output pulse.

5. An electrohydraulic velocity servoactuator according to claim 4 wherein said shut-off means includes null detector means arranged to sense any command signal and phase angle detector means arranged to sense the phase of the output of said pick-up winding.

6. An electrohydraulic velocity servoactuator according to claim 5 wherein said shut-off means further includes first flip-flop logic means responsive to the outputs of said null detector means and said phase angle detector means.

7. An electrohydraulic velocity servoactuator according to claim 6 wherein said shut-off means further includes second flip-flop logic means responsive to the outputs of said first flip-flop logic means and said clock means, said command counter means is responsive to the outputs of said second flip-flop logic means and said clock means, and said one shot means is responsive to the outputs of said second flip-flop logic means and said command counter means.

8. In a rolling mill including work rolls between which a workpiece is moved for reduction of its thickness, the improvement of velocity servoactuator means which comprises screwdown hydraulic actuator means for positioning said rolls, and electrohydraulic servovalve means for controlling the flow of fluid with respect to said actuator means and controlled by an electrical command signal selectively derived from either automatic gauge control means or manual control means, said servovalve means being operatively associated with said actuator means to provide a velocity loop for producing a correction of the position of said rolls at a velocity proportional to the magnitude of said command signal.

9. A rolling mill according to claim 8 wherein said velocity servoactuator means includes velocity sensing means arranged to produce an electrical velocity signal responsive to the velocity of the output motion of said actuator means, means summing said velocity signal and said command signal to provide an electrical error signal, and amplifier means arranged to receive said error signal and operate said servovalve means.

10. A rolling mill according to claim 8 which further comprises position holding means operatively associated with said velocity loop to provide a position holding loop for holding the position of said rolls when said command signal is zero.

l l. A rolling mill according to claim 10 wherein said velocity sensing means comprises tachometer means llv including a tachometer rotor rotated in response to the output motion of said actuator means, and said position holding means includes resolver means having a resolver rotor and a stator, said resolver rotor also being rotated in response to the output motion of said actuator means, said rotor including a pick-up winding, said stator including two phase windings, high frequency clock means, reference counter means operatively associated with said clock means and said two phase windings and arranged to be driven by said clock means to provide a lower carrier frequency with a two phase output which energizes said two phase windings, command counter means operatively associated with said clock means and arranged to produce a phase output at the same frequency as said carrier frequency, sampling demodulator means comparing the phase of the electrical output of said pick-up winding with said phase output of said command counter means and arranged to produce a polarity-sensitiveroutput that is proportional to the difference between such phases but zero when said phases are equal, means arranged to feed said polarity-sensitive output to said velocity loop, and shutoff means arranged to shut off said command counter means when a command signal exists and thereby provide no such proportional polarity-sensitive output.

12. In a rolling mill including work rolls between which a workpiece is moved for reduction of its thickness, screwdown hydraulic actuator means for positioning said rolls, and a screwdown control system including electrohydraulic servovalve means for controlling the flow of fluid with respect to said actuator means and controlled by an electrical command signal selectively derived from either automatic gauge control means or manual control means, the improvement which comprises velocity mode means operatively associated with said servovalve means to provide a velocity loop for producing a correction of the position of said rolls at a velocity proportional to the magnitude of said command signal.

137 A rolling mill according to claim 12 wherein said velocity mode means includes velocity sensing means arranged to produce an electrical velocity signal responsive to the velocity of the output motion of said actuator means, means summing said velocity signal and said command signal to provide an electrical error signal, and amplifier means arranged to receive said error signal and operate said servovalve means.

14. A rolling mill according to claim 12 which further comprises position holding mode means operatively associated with said velocity loop to provide a position holding loop for holding the position of said rolls when said command signal is zero. 

1. In an electrohydraulic servoactuator for controlling the velocity of a load, including hydraulic actuator means for positioning said load and electrical input servovalve means For controlling the flow of fluid with respect to said actuator means, the improvement which comprises zero velocity position holding means including phase generating means movable in response to the output motion of said actuator means, said phase generating means including at least two phase windings and a pick-up winding having a signal output whose phase varies with load position.
 2. An electrohydraulic velocity servoactuator according to claim 1 which further comprises tachometer means including a tachometer rotor rotated in response to the output motion of said actuator means, and said position holding means includes resolver means having a resolver rotor and a stator, said resolver rotor also being rotated in response to the output motion of said actuator means said rotor including a pickup winding, said stator including two phase windings, high frequency clock means, reference counter means operatively associated with said clock means and said two phase windings and arranged to be driven by said clock means to provide a lower carrier frequency with a two phase output which energizes said two phase windings, command counter means operatively associated with said clock means and arranged to produce a phase output at the same frequency as said carrier frequency, sampling demodulator means comparing the phase of the electrical output of said pick-up winding with said phase output of said command counter means and arranged to produce a polarity-sensitive output that is proportional to the difference between such phases but zero when said phase are equal, means arranged to feed said polarity-sensitive output to said servovalve means, and shut-off means arranged to shut off said command counter means when the command signal exists and thereby provide no such proportional polarity-sensitive output.
 3. An electrohydraulic velocity servoactuator according to claim 1 wherein said position holding means includes resolver means having a stator including two phase windings and also having a rotor operatively responsive to output motion of said actuator means and including a pick-up winding, high frequency clock means, reference counter means operatively associated with said clock means and said two phase windings and arranged to be driven by said clock means to provide a lower carrier frequency with a two phase output which energizes said two phase windings, command counter means operatively associated with said clock means and arranged to produce an output at the same frequency as said carrier frequency but having a phase differing by any cycle of said clock means, phase sensitive demodulator means comparing the phase of the electrical output of said pick-up winding with said phase output of said command counter means and arranged to produce a polarity-sensitive output that is proportional to the difference between such phases, means arranged to feed said polarity-sensitive output to said servovalve means, and shut-off means arranged to shut off said command counter means when a command signal exists and thereby provide no such proportional polarity-sensitive output.
 4. An electrohydraulic velocity servoactuator according to claim 3 which further comprises one shot means operatively associated with said command counter means and arranged to produce an output pulse having the same phase and frequency as the output from said command counter means, and wherein said phase sensitive demodulator means includes sampling demodulator means controlled by said output pulse.
 5. An electrohydraulic velocity servoactuator according to claim 4 wherein said shut-off means includes null detector means arranged to sense any command signal and phase angle detector means arranged to sense the phase of the output of said pick-up winding.
 6. An electrohydraulic velocity servoactuator according to claim 5 wherein said shut-off means further includes first flip-flop logic means responsive to the outputs of said null detector means and said phase angle detector means.
 7. An electrohydraulic vElocity servoactuator according to claim 6 wherein said shut-off means further includes second flip-flop logic means responsive to the outputs of said first flip-flop logic means and said clock means, said command counter means is responsive to the outputs of said second flip-flop logic means and said clock means, and said one shot means is responsive to the outputs of said second flip-flop logic means and said command counter means.
 8. In a rolling mill including work rolls between which a workpiece is moved for reduction of its thickness, the improvement of velocity servoactuator means which comprises screwdown hydraulic actuator means for positioning said rolls, and electrohydraulic servovalve means for controlling the flow of fluid with respect to said actuator means and controlled by an electrical command signal selectively derived from either automatic gauge control means or manual control means, said servovalve means being operatively associated with said actuator means to provide a velocity loop for producing a correction of the position of said rolls at a velocity proportional to the magnitude of said command signal.
 9. A rolling mill according to claim 8 wherein said velocity servoactuator means includes velocity sensing means arranged to produce an electrical velocity signal responsive to the velocity of the output motion of said actuator means, means summing said velocity signal and said command signal to provide an electrical error signal, and amplifier means arranged to receive said error signal and operate said servovalve means.
 10. A rolling mill according to claim 8 which further comprises position holding means operatively associated with said velocity loop to provide a position holding loop for holding the position of said rolls when said command signal is zero.
 11. A rolling mill according to claim 10 wherein said velocity sensing means comprises tachometer means including a tachometer rotor rotated in response to the output motion of said actuator means, and said position holding means includes resolver means having a resolver rotor and a stator, said resolver rotor also being rotated in response to the output motion of said actuator means, said rotor including a pick-up winding, said stator including two phase windings, high frequency clock means, reference counter means operatively associated with said clock means and said two phase windings and arranged to be driven by said clock means to provide a lower carrier frequency with a two phase output which energizes said two phase windings, command counter means operatively associated with said clock means and arranged to produce a phase output at the same frequency as said carrier frequency, sampling demodulator means comparing the phase of the electrical output of said pick-up winding with said phase output of said command counter means and arranged to produce a polarity-sensitive output that is proportional to the difference between such phases but zero when said phases are equal, means arranged to feed said polarity-sensitive output to said velocity loop, and shut-off means arranged to shut off said command counter means when a command signal exists and thereby provide no such proportional polarity-sensitive output.
 12. In a rolling mill including work rolls between which a workpiece is moved for reduction of its thickness, screwdown hydraulic actuator means for positioning said rolls, and a screwdown control system including electrohydraulic servovalve means for controlling the flow of fluid with respect to said actuator means and controlled by an electrical command signal selectively derived from either automatic gauge control means or manual control means, the improvement which comprises velocity mode means operatively associated with said servovalve means to provide a velocity loop for producing a correction of the position of said rolls at a velocity proportional to the magnitude of said command signal.
 13. A rolling mill according to claim 12 wherein said velocity mode means incLudes velocity sensing means arranged to produce an electrical velocity signal responsive to the velocity of the output motion of said actuator means, means summing said velocity signal and said command signal to provide an electrical error signal, and amplifier means arranged to receive said error signal and operate said servovalve means.
 14. A rolling mill according to claim 12 which further comprises position holding mode means operatively associated with said velocity loop to provide a position holding loop for holding the position of said rolls when said command signal is zero. 